Electron tube and circuitry apparatus matched for low rf losses



July 16. 1968 E. LOVE ETAL 3,393,340

ELECTRON TUBE AND CIRCUITRY APPARATUS MATCHED FOR LOW RF LOSSES Filed June 26, 1965 5 Sheets-Sheet 1 INVENTORS KENNETH E. LOVE ROBERT C. MORWOOD JAMES R POLESE wwil, dim? W ATTORNEYS July 16. 1968 ELECTRON TUBE AND CIRCUITRY AFFARATUS MAT Filed June 28, 1965 K. E. LOVE ETAL 3,393,340

SHED FOR LOW RF LOSSES 5 Sheets-Sheet :4;

INVENTORS KENNETH E. LOVE ROBERT C. MORWOOD JAMES R POLESE y 1958 K. E. LOVE ETAL 3,393,340

ELECTRON TUBE AND CIRCUITRY APPARATUS MATCHED FOR LOW RF LOSSES Filed June 23, 1965 5 Sheets-Sheet 3 INVENTORS KENNETH E. LOVE ROBERT c. MORWOOD JAMES R POLESE m di P. mm ATTORNEYS United States Patent 3,393,340 ELECTRON TUBE AND 'CIRCUITRY APPARATUS MATCHED FOR LOW RF LOSSES Kenneth E. Love, Belmont, Robert C. Morwood, Redwood City, and James P. Polese, Menlo Park, Calif., assignors, by mesne assignments to Varian Associates, a corporation of California Filed June 28, 1965, Ser. No. 467,202 18 Claims. (Cl. 313-283) In general, the purpose of the invention is to provide an improved negative grid tube design which will permit manufacture of tubes that will operate at higher power at high frequency, for example tubes that will operate at over 75 watts CW power output in the 2100 me. frequency region.

One of the limitations on performance of negative grid tubes in the frequency region under consideration is that an appreciable part of the microwave circuit is inside the tube envelope. Conventional tube designs, except those with very low power, present series field discontinuities when installed in conventional cavity circuitry.

Accordingly, an object of the present invention is to provide an improved negative grid tube designed so that the tube itself provides minimum disturbance of RF fields which exist during operation of the tube.

Another object of the invention is to provide an improved negative grid tube and circuitry apparatus designed so that the tube will be connected to the circuitry with minimum disturbance of RF fields.

A related object of the invention is to provide an improved negative grid tube which is adapted for connection to a coaxial line type of circuitry and also to a waveguide type of circuitry.

An additional object of the invention is to provide improved ceramic-to-metal sealing constructions which facilitate design of a tube meeting the previously stated objectives.

In order to provide increased power a preferred form of the invention involves an annular cathode, and therefore a further object of the invention is to provide an improved construction for an annular heater-cathode package.

These and other objects and features of advantage will become more apparent from the following detailed description wherein reference is made to the accompanying drawings in which:

FIGURE 1 is a cross-sectional view on the center line of a planar electrode negative grid tube according to the invention;

FIGURE 2 is an enlarged sectional view through one side of the annular cathode.

FIGURE 3 is a view showing the tube of FIGURE 1 in coaxial line circuitry. The tube in FIGURE 3 is shown on a smaller scale than in FIGURE 1 and the center parts of the tube are shown in elevation; and

FIGURE 4 is a view similar to FIGURE 3 but showing the tube in waveguide type external circuitry.

Referring to the drawings in more detail the tube comprises an anode 1, a cathode 2, and a grid 3. In order to provide a high power design, the electrodes are made annular in the preferred embodiment. One of the principal limitations on the size of a conventional planar triode is grid cooling. Cooling is mainly by thermal conduction and, since grid dissipation is uniform over the grid area, the hottest point is the center of the grid. For a given maximum allowable grid temperature and a given current density there is a maximum limit on grid diameter. In cooling the center of the grid by thermal conduction, heat will flow along a grid radius. With an annular configuration, however, grid area can be increased and, since there is no dissipation in the center of the grid, the heat "ice conduction path can be made very short. More specifically, the width of the grid annulus is the length of the heat path. The width of the grid annulus can be maintained constant and more area obtained by increasing the diameter of the annulus. Thus the cathode area and the length of the grid thermal conduction path may each be set independently. As a result the grid dissipation rating can be increased to several times that of the planar triodes with solid disc cathodes.

The anode comprises a core portion 4 having a threaded portion 5 projecting from the end thereof. In order to be able to seal a ceramic envelope to the anode, the core portion 4 is surrounded by a thin metal sleeve 6. The sleeve 6 is brazed to the core 4 adjacent the ends thereof. The middle part of the core is made with a reduced diameter portion 7 so that it is peripherally separated from the sleeve 6 by an annular space 8. The diameter of the threaded portion 5 is less than the diameter of the sleeve 6 to provide an annular abutment shoulder 9. The core 4 is preferably made of copper to provide good thermal conductivity and the sleeve 6 is preferably made of Kovar to more closely approximate the coefficient of expansion of the dielectric envelope, as will be more fully described hereinafter.

The anode has an annular electron receiving surface 11 surrounding a central circular recess which extends partway toward the threaded end of the core 4. The outer part of the wall of the circular recess is a straight cylindrical portion 12 to provide a symmetrical counter part for the annular cathode. The next wall portion of the central recess tapers inwardly as at 13, followed by another straight cylindrical portion 14, and terminating with another tapered portion 15. The purpose of the construction is to provide a recess which is progressively smaller away from the electron receiving surface of the anode. The end of the anode could be made solid but such a construction would present too much capacitance with the adjacent electrode. Also the circular recess could be made with a straight cylindrical sidewall throughout its entire length but this construction would remove too much of the thermally-conductive mass of the core. In order to facilitate removal of gas from the space 8, the core 4 and sleeve 6 are provided with bores 16.

The cathode structure comprises an annular member 20 having a U-shaped cross-section and providing an electron emissive surface 21 of annular configuration. The surface 21 is preferably coated with a conventional electron emissive material. The member 20 is preferably made of nickel and is heated by means of a toroidal heater coil 22. The heater coil 22 is held firmly in position yet spaced from the member 20 by means of four small ceramic rings 23. After the heater 22 and rings 23 have been inserted in the member 20 the open hand of member 20 is closed by a thin nickel washer 24 which is spot-welded in place.

The cathode member 20 is mounted on a cylindrical support in the form of a thin wall Kovar cylinder 27 which also serves as a heat dam. The cylindrical support is reinforced and extended -by a slightly thicker Kovar cylinder 28. The cylinder 27 can be attached to member 20 and to the cylinder 28 by means of spot-welding. The outer end of the thin wall cylinder 28 is brazed to a relatively thick wall member 29 also preferably of Kovar. The member 29 is provided with an externally-threaded portion 30 and a substantially undercut portion 31 for purposes to be hereinafter described in detail. It will be noted that the threaded portion 30 is of smaller diameter than the cylindrical support 27, 28 to provide an annular abutment shoulder 32. A dielectric envelope cylinder 34 preferably of high alumina ceramic is metalized by conventional procedures and brazed at one end to the wall member 29. The other end of cylinder 34 is brazed to a support sleeve 35 which is preferably made of Kovar. The member 35 is provided with an externally-threaded portion 33 and a substantially undercut portion for purposes to be hereinafter described in detail. A copper exhaust tubulation 36 is brazed in the aperture in sleeve 35. After the tube has been evacuated the exhaust tubulation is pinched and sealed as at 37 in conventional manner. Finally, a brass closure cup 38 is screwed on the support sleeve 35 and carries an internally-threaded connector 39 for one lead of the heater.

The grid 3 comprises a conventional wire mesh structure brazed to a metal grid ring 42. It is necessary that an annular terminal for the grid be extended outwardly of the tube envelope. Any conventional construction can be employed for this purpose. One very suitable construction although not claimed as part of the present invention comprises two sealing rings 43 and 44, preferably of Kovar. The sealing rings are brazed to annular dielectric envelope wall sections 45 and 46, respectively, The wall sections 45 and 46 are preferably high alumina ceramic and are metalized in conventional manner to permit brazing to the sealing rings. The annular ceramic member 45 is also brazed to the thin sleeve 6, in the area where the sleeve is spaced from the anode core 4. The ceramic member 46 is brazed to the thin wall cylindrical support 28. The opposing faces of rings 43 and 44 are so shaped that when the radially-inner portions of the rings abut the grid ring 42, the radially-outer portions of the sealing rings 43 and 44 are spaced slightly apart. During the final processing of the tube the sealing rings 43 and 44 are pressed together adjacent their outer rims and are then welded together. In this manner, the grid ring 42 is clamped firmly in place between the sealing rings. Although the grid 3 is shown as having a complete circle shape the center portion thereof could, of course, be omitted. The only electronically required part of the grid is the annular portion interposed between the annular cathode electron-emitting surface 21 and the annular anode electron-receiving surface 11.

In a preferred embodiment of the invention a Kovar support tube 48 is brazed in the support sleeve 35. A heater tab 49 is spot-welded to the post 48, and one lead 50 for the heater coil 22 is spot-welded to the tab 49. A second heater tab 51 is spot-welded to the cylinder 28 and a second heater lead 52 is spot-welded to the tab 51. A cup-shaped member 55 of zirconium getter material is preferably supported at the top of the post 48.

The reason for the multi-piece anode construction will now be explained in more detail. The copper anode core 4 has a much greater coeflicient of expansion than the ceramic envelope section 45. As a result of this mismatch it is not possible to braze the ceramic section 45 directly to the copper core 4. The problem has been solved according to the invention by the provision of the relatively thin wall sleeve 6 which is separated from the core 4 by the space 8. Thus, the difference in coefficient of expansion between the ceramic section 45 and the sleeve 6 can be accommodated by slight deformation of the sleeve. In addition, the arrangement whereby sleeve 6 is a separate member from the core 4 makes is possible to construct the sleeve out of a different material such as Kovar which more nearly matches the coeflicient of expansion of the ceramic section 45. As regards the ceramic section 46 it will be noted that it is attached to a relatively thin and therefore deformable cylindrical member 28. The relatively thick metal ring members 29 and 35 are undercut at 31 and 40 so that they will be weakened enough to deform slightly rather than destroy the ceramic ring 34 as a result of different thermal expansion and contraction.

Referring now to FIGURE 3 it will be seen that the tube is particularly adapted to be associated with microwave circuitry in the form of a coaxial line type of resonant cavity 60 associated with the cathode and a coaxial line type of resonant cavity 61 associated with the anode. Resonator 60 comprises a cylindrical inner conductor 62 which is threaded at its inner end for engagement with the threaded portion 30 on the metal member 29 of the tube, with the inner end of the, cylinder 62 engaging the abutment shoulder 32 of the tube. The cavity 60 has a cylindrical outer conductor 64 which is coaxial with the inner conductor 62. In order to make aclosed cavity 60 the conductors 62 and 64 are connected by an end wall 65. In order to provide a DC lead to one side of the heater, a conductor rod 66 is threaded into the connector 39. Rod 66 is insulated from the cavity 60 by a dielectric bushing 67. It will 'be noted that the diameter of the cylinder 62 is the same as the diameter of the cylindrical supports 28, 27 of the tube which is in turn of the same diameter as the cathode emissive surface 21. Thus, the RF current can follow a straight line path from the conductorl62 to the cathode surface 21. The outer cylindrical conductor is provided at its end with an annular ring of contact fingers 69 which engage the sealing ring 44. It will be understood that. the sealing rings 43 and 44 together with the grid ring 42 form a grid terminal ring assembly. It will also be understood that the cavity resonator 60 can be provided with conventional tuning plungers, input coupling devices and blocking capacitors (not shown).

The anode end of the tube is similarly adapted for matched coupling with a coaxial line type of resonant cavity 61. The cavity 61 has a center conductor 72 which is threaded at its inner end for engagement with the threaded portion 5 0n the tube, with the inner end of the conductor 72 engaging the abutment shoulder 9 on the tube. The output cavity 61 has a cylindrical outer conductor 74 which is coaxial with the inner conductor 72.

In order to make a closed cavity 61 the conductors 72 and 74 are connected by an end wall 75. As in the case of cavity 60', the cavity 61 can be provided with conventional tuning, coupling and blocking means (not shown). The inner end of the outer conductor 74 is provided with a ring of contact fingers 76 which engage the sealing ring 43. -It should be noted that the inner conductor 72 is of the same diameter as the wall of the anode sleeve 6 which is of the same diameter as the electron-receiving surface 11 of the anode. Thus, the RF current can follow a direct straight line path from the conductor 72 to the anode and in particular is not interrupted by any complicated sealing ring structure between the ceramic section 45 and the anode sleeve 6.

Referring to FIGURE 4 it will be seen that the tube is also particularly adapted for use in connection with waveguide type resonant cavities. More specifically, a resonant cavity 77 is associated with the cathode end of the tube and a resonant cavity 78 of the waveguide type is associated with the anode end of the tube. The resonant cavity 77 comprises an end wall 79 having a central aperture '80 therein. The aperture 80 is slightly larger than the diameter of the threaded portion 30 of the tube in order to fit over said threads. However, the aperture 80 is of smaller diameter than the outer diameter of the annular shoulder 32 of the tube so that the wall 79 engages the shoulder 32 and can be securely held in place by a nut 81. The resonant cavity 77 includes a second wall 82 which is parallel to the wall 79 and is electrically connected thereto by a peripheral Wall 83. The peripheral wall 83 can be rectangular or circular but is preferably rectangular to provide the usual Waveguide type of cavity. The wall 82 is centrally apertured and provided with a ring of contact fingers 85 which engage the sealing ring 44. As in the case of the coaxial line cavity,'it will be seen that the current flowing from the cavity wall 79 can proceed along a straight direct path to the cathode emissive surface 21.

In similar manner the anode end of the tube is adapted for matched cooperation with a waveguide type of cavity 78. More specifically, the cavity 78 comprises a wall 86 provided with a central aperture 87. The aperture 87 is of slightly larger diameter than the threaded portion 5 on the tube so as to fit thereover and is of smaller diameter than the outer diameter of the annular shoulder 9 so that the wall 86 rests on shoulder 9 and can be held securely in place by a nut 88. As in the case of cavity 77, the cavity 78 includes a second wall 89 parallel to the wall 86 and electrically connected thereto by a peripheral wall 90. The wall 89 is centrally aperturcd and providedwith a ring of contact fingers 91 which engage the sealing ring 43. It will be understood that the waveguide cavity resonators 77 and 78 can each be provided with conventional coupling, tuning and blocking means (not shown). It should also be understood that one end of the tube can be connected in a coaxial line type of cavity as shown in FIGURE 3 and the other end of the tube connected in a waveguide type of cavity as shown in FIGURE 4. It should be noted that the diameter of cylinders 6 and 27 are substantially the same, and that the distance from grid 3 to shoulder 9 is substantially the same as the distance from grid 3 to shoulder 32. In this way the input and output resonant cavities can have substantially the same dimensions and therefore can be more. efiiciently made in greater quantities.

Although specific details of the present invention have been shown and described herein, it should be understood that the invention encompasses all changes and modifications coming within the spirit and scope of the appended claims.

What is claimed is:

1. An electron tube comprising an anode having a planar electron-receiving surface inside the tube, said anode having a cylindrical portion with a sidewall surface having the same diameter as said planar electron-receiving surface, a threaded portion projecting from said cylindrical portion opposite said electron-receiving surface, said threaded portion having-a smaller diameter than said cylindrical portion to provide an annular abutment shoulder, a first annular envelope wall section of dielectric material sealed to said cylindrical wall of the anode, a planar grid parallel to said electron-receiving surface of the anode, an annular terminal connected to said grid and sealed to said first annular dielectric wall section, a cathode having a planar electron emissive surface parallel to said grid and on the side of the grid opposite the anode, a cylindrical support connected to said cathode and having the same diameter as said emissive surface, a threaded portion projecting from said cylindrical support opposite said emissive surface and having a smaller diameter than V the cylindrical support to provide an annular abutment shoulder, and a second annular dielectric envelope wall section of dielectric material sealed to said cylindrical support and to said grid terminal.

2. An electron tube as claimed in claim 1 in which said anode comprises a core portion and a thin sleeve around the core, said sleeve being secured to the core at positions spaced along the core, said first annular dielectric Wall section being sealed to said sleeve intermediate said spaced positions, and said core portion being of small enough diameter intermediate said spaced positions to be peripherally separated from said sleeve.

3. An electron tube as claimed in claim 1 in which said cylindrical support is a thin wall sleeve and said threaded portion projecting therefrom is a relatively thick wall metal member, and said thick Wall member is provided with an undercut between the end of the threads thereon and said annular shoulder, a dielectric cylinder sealed to said thick wall member, a metal support sleeve sealed to said dielectric cylinder, and an exhaust tube sealed in an aperture in said support sleeve.

4. An electron tube as claimed in claim 1 in which said cathode comprises an annular metal member having a U-shaped cross-section, a closure ring over the open end of the U-shaped member to form an annular space having a rectangular cross-section, a toroidal heater coil in said annular space in the U-shaped member, and a ceramic ring in each of the four corners of said annular space holding the heater spaced from the walls of the U-shaped member.

5. An electron tube as claimed in claim 1 further comprising a coaxial line cavity resonator having coaxial inner and outer conductive cylinders, said inner cylinder having internal threads at one end engaging one of said threaded portions with the end of the inner cylinder being in engagement with the adjacent one of said abutment shoulders, the outside diameter of said inner cylinder being the same as the outside diameter of said adjacent abutment shoulder, and said outer cylinder being electrically connected to said grid terminal.

6. An electron tube as claimed in claim 1 further comprising a waveguide cavity resonator having spaced walls forming a rectangular cavity, one of said walls having an aperture therein fitting over one of said threaded portions and smaller than the adjacent abutment shoulder whereby said wall engages said adjacent shoulder, a nut threaded on said one threaded portion and forcing said one resonator wall against said adjacent shoulder, and a second one of said walls of the resonator being electrically connected to said grid terminal.

7. An electron tube as claimed in claim 1 wherein the diameter of said cylindrical wall of the anode is substantially the same as the diameter of said cylindrical support for the cathode, and the distance from said grid to one of said abutment shoulders is substantially equal to the distance from said grid to the other of said abutment shoulders.

8. An electron tube as claimed in claim 1 in which said grid and said emissive surface are annular.

9. An electron tube as claimed in claim 8 in which said anode comprises a core portion having a circular recess in the end facing said grid whereby said electronreceiving surface is annular, and the diameter of said circular recess is progressively smaller away from said grid to provide a good heat conducting path and yet maintain low grid-to-anode capacitance.

10. An electron tube comprising an envelope containing electrodes including an anode and a cathode, said anode comprising a core portion and a thin sleeve around the core, said sleeve being secured to the core at positions spaced along the core, said envelope comprising an annular dielectric wall section, said dielectric wall section being sealed to said sleeve intermediate said spaced positions, and said core being of small enough diameter intermediate said spaced positions to be peripherally separated from said sleeve.

11. An electron tube comprising an envelope containing electrodes including an anode and a cathode, said cathode comprising an electron emissive surface, a thin wall cylindrical support for said emissive surface, a relatively thick annular metal wall member connected to said cylindrical support remote from said emissive surface, said thick wall member being provided with external threads thereon and a shoulder of larger diameter than the threads between the threads and said emissive surface, said thick wall member being provided with an undercut between the end of the threads and said shoulder, a dielectric cylinder sealed to said wall member, a metal support sleeve sealed to said dielectric cylinder, and an exhaust tube sealed in an aperture in said support sleeve, said envelope comprising an annular dielectric wall section, and said dielectric wall section being sealed to said thin wall cylindrical support intermediate said emissive surface and said thick wall member.

12. An electron tube comprising an envelope containing electrodes including a planar cathode and a planar anode, said cathode comprising an annular metal member having a U-shaped cross-section, a closure ring over the open end of the U-shaped member to form an annular space having a rectangular cross-section, a heater coil in said annular space in the U-shaped member, and a ceramic ring in each of the four corners of said annular space holding the heater spaced from the walls of the U-shaped member.

13. An electron tube comprising an envelope containing planar electrodes including an anode and a cathode, said anode having an electron-receiving surface inside the tube and a cylindrical portion with a sidewall surface having the same diameteras said electron-receiving surface, a threaded portion projecting from said cylindrical portion opposite said electron'receiving surface, said threaded portion having a smaller diameter than said cylindrical portion to provide an annular abutment shoulder, and said envelope comprising an annular dielectric wall section sealed to said cylindrical wall'o'f the anode intermediate said annular shoulder and said electronreceiving surface.

14. Electron tube appearatus comprising an envelope containing planar electrodes including an anode and a cathode, said anode having an electron-receiving surface inside the tube and a cylindricalportion with a sidewall surface having the same diameter as said electron-receiving surface, a threaded portion projecting from said cylindrical portion opposite said electron-receiving surface, said threaded portion having a smaller diameter than said cylindrical portion to provide an annular abutment shoulder, said envelope comprising an annular dielectric wall section sealed to said cylindrical wall of the anode intermediate said annular shoulder, and said electronreceiving surface, an external circuit member connected to said tube and comprising a conductor having a cylindrical side wall surface, said external circuit member having internal threads at one end thereof engaging said threads on the anode with the end of the circuit member engaging said abutment shoulder, and the diameter of said cylindrical surface of the circuit member being the same as the diameter of said cylindrical sidewall surface of the anode.

15. An electron tube comprising an envelope containing planar electrodes including an anode and a cathode, said cathode comprising a circular electron emissive surface facing said anode, a cylindrical support for said electron emissive surface and having the same diameter as said emissive surface, a threaded portion projecting from said cylindrical support opposite said emissive surface and having a smaller diameter than the cylindrical support to provide an annular'abntment shoulderj'and said envelope comprising an annular dielectric wall section sealed to said cylindrical support intermediate said electron emissive surface and said abutment shoulder.

16. Electron tube apparatus comprising an envelope containing planar electrodes including an anode and a cathode, said cathode comprising a circulaf electron emissive surface facing said anode, a cylindrical support for said electron emissive surface andhavingathe same diameter as said emissive surface, ;a threaded portion projecting from said cylindrical support opposite said emissive surface and having a smaller diameter than the cylindrical support to provide anannular abutment shoulder, said envelope comprising an annular dielectric wall section sealed to said cylindrical support intermediate said electron emissive surface and said abutmt i, anexternal circuit member connected to said tube and comprising a conductor having a cylindrical sidewall surface, .said external circuit member having internalthreads at one end thereof engaging said threaded portion with the end of the circuit member engaging said abutment shoulder, and the diameter of said cylindrical surface of the circuit memberbeing the same as the diameter of said cylindrical support for the cathode.

17. An electron tube having an envelope comprisinga relatively thick metal member, a relatively thin wall metal cylinder bonded to said thick metal member at one position and spaced from said thick metal member at a second position, and a circular dielectric wall section having a cylindrical surface thereof bonded to said thin wall cylinder at said second position. I

18. An electron tube having an envelope comprising a cylindrical metal member having a relativelythick section, a relatively thin and threaded section spaced from the thick section, a necked down portion betweensaid thick section and said threaded section, and a dielectric wall member bonded to said threaded section on the side opposite the threads.

No references cited.

JOHN W. HUCKERT, Primary Examiner.

A. J. JAMES, Assistant Examiner. 

1. AN ELECTRON TUBE COMPRISING AN ANODE HAVING A PLANAR ELECTRON-RECEIVING SURFACE INSIDE THE TUBE, SAID ANODE HAVING A CYLINDRICAL PORTION WITH A SIDEWALL SURFACE HAVING THE SAME DIAMETER AS SAID PLANAR ELECTRON-RECEIVING SURFACE, A THREADED PORTION PROJECTING FROM SAID CYLINDRICAL PORTION OPPOSITE SAID ELECTRON-RECEIVING SURFACE, SAID THREADED PORTION HAVING A SMALLER DIAMETER THAN SAID CYLINDRICAL PORTION TO PROVIDE AN ANNULAR ABUTMENT SHOULDER, A FIRST ANNULAR ENVELOPE WALL SECTION OF DIELECTRIC MATERIAL SEALED TO SAID CYLINDRICAL WALL OF THE ANODE, A PLANAR GRID PARALLEL TO SAID ELECTRON-RECEIVING SURFACE OF THE ANODE, AN ANNULAR TERMINAL CONNECTED TO SAID GRID AND SEALED TO SAID FIRST ANNULAR DIELECTRIC WALL SECTION, A CATHODE HAVING A PLANAR ELECTRON EMISSIVE SURFACE PARALLEL TO SAID GRID AND ON THE SIDE OF THE GRID OPPOSITE THE ANODE, A CYLINDRICAL SUPPORT CONNECTED TO SAID CATHODE AND HAVING THE SAME DIAMETER AS SAID EMISSIVE SURFACE, A THREADED PORTION PROJECTING FROM SAID CYLINDRICAL SUPPORT OPPOSITE SAID EMISSIVE SURFACE AND HAVING A SMALLER DIAMETER THAN THE CYLINDRICAL SUPPORT TO PROVIDE AN ANNULAR ABUTMENT SHOULDER, AND A SECOND ANNULAR DIELECTRIC ENVELOPE WALL SECTION OF DIELECTRIC MATERIAL SEALED TO SAID CYLINDRICAL SUPPORT AND TO SAID GRID TERMINAL. 